The invention relates to a process for manufacturing clad strips in which a core strip, in particular of metal, is fed to a roll gap formed by at least two rolls and is clad with a cladding strip on at least one side, and relates also to a device for this purpose.
Processes for cladding in particular metal strips find application for example when a lower grade core material is to be provided with a thin layer of high grade material. The lower quality of the core material can be due to the alloy used. This includes alloys which contain a large fraction of recycled scrap as the amount of the various elements present and their distribution vary considerably. The lower quality of the core material can also be due to the irregular roughness of the as-cast surface, for example as in strips produced on continuous casting machines.
As the surface properties of strip material are often decisive in many applications, cladding processes find frequent use.
Whereas, for example, cast slabs have cladding sheets of plates laid or welded on to them and are joined to them by pre-heating and rolling, in the case of cast strip one requires a synchronously running facility for feeding the cladding material to the cast strip. As such it is of no consequence whether these strips are rolled out from a coiler used as an intermediate storage means, or come from the strip casting unit directly. Known for the cladding of such strips are arrangements in which a cladding strip is led via feeding rolls to a unit where it is laid on top of the core material. After passing through a preheating zone, the two materials enter a roll gap of a hot rolling mill. It has been found disadvantageous here that the cladding strip, due to its relatively long contact time with the core material, enters the roll gap at approximately the same temperature as the core material. The hot forming characteristics of both materials are about the same in that case with the result that no relative movement takes place between the core and the cladding material i.e. a movement which would help achieve optimal welding together of the two materials. Also, in high grade cladding material undesirable grain growth takes place, an effect which becomes all the more undesirable the longer the duration of contact between the core and cladding material in the area between the point of first contact and the roll gap. High grade products on the other hand are exactly those products which require a very fine grain.
Furthermore it has been noted that distortion, at least of the cladding material, takes place between the point of first contact and the roll gap due to differences in thermal expansion so that it is no longer possible to achieve proper alignment of the two component materials, and blisters can form due to the entrapment of air i.e. an effect which has a negative effect on the bond strength due to a reduction of the area where the core and cladding materials are bonded together.
For example in the manufacture of strips made of an aluminum alloy with a high concentration of magnesium the as-cast strip acquires an oxide skin on route from the caster to the hot rolling mill which makes it difficult to bond the cladding material to the core material. The greater this distance, especially if a preheating zone is provided, the more disturbing is the growth of the oxide skin. This oxide skin should therefore be removed as much as possible in order to allow diffusion bonding between the materials in the roll gap.
The object of the present invention is therefore to develop a process and device of the kind mentioned at the start wherein the above mentioned disadvantages are avoided. Furthermore the surface properties of the end product and the bond strength between the cladding and the core material should be improved.